This invention relates to recovery of unreacted adiponitrile (ADN) in a mixture of ADN, aminocapronitrile (ACN) and hexamethylenediamine (HMD) that is formed by the partial hydrogenation of ADN. The ADN can come from the hydrocyanation of butadiene (BD), hydrodimerization of acrylonitrile, aminolysis of adipic acid or any other technique. It is known that ADN can by partially hydrogenated to HMD and ACN by reacting the ADN with hydrogen in the presence of a heterogeneous catalyst, notably iron or Raney cobalt. Such hydrogenations result in the production of undesirable by-products, including bishexamethylenetriamine (BHMT) and high boiling tars.
If such a process were to be run in a way that unreacted ADN is recycled back to the hydrogenation reactor, the BHMT would be recycled back with it and, as a result, the BHMT would be expected to increase over time, leading to an unacceptable level of accumulation in the system.
After the partial hydrogenation reaction, the resulting ACN and HMD need to be separated from the unreacted ADN and from each other. The separations can be effected using fractional distillation. During such distillation an undesirable byproduct, 2-cyanocyclopentylideneimine (CPI) results from the isomerization of ADN under alkaline conditions that exist during distillation.
One way to separate the CPI from the ADN is by simple, fractional distillation, in which the CPI is obtained as a distillate. However, such simple fractional distillation, if practiced using conventional operating conditions, i.e., less than 20 mm Hg head pressure, would not allow BHMT to be separated along with the CPI from the ADN.
The present invention provides a process in which BHMT and CPI can be separated together from ADN in a mixture comprising all three components.
In particular, the present invention is a process for recovering substantially pure ADN from a feed mixture comprising BHMT, ADN, CPI and high boiling tars, said process comprising the following sequential steps:
(1) distilling the mixture in an ADN Recovery Distillation Column having a head pressure in the range of 2 to 150 mm Hg to produce a substantially tar-free distillate comprising at least 70% of the ADN in the feed mixture, BHMT and CPI, and a bottoms product comprising substantially all of the tars and the remainder of the ADN in the feed mixture; and
(2) redistilling the distillate from step (1) in an ADN Refining Distillation Column having a head pressure in the range of 20 to 150 mm Hg, which causes a minimum temperature azeotrope to form between ADN and BHMT, to produce (i) a distillate containing the majority of the BHMT and CPI present in the distillate from step (1), and (ii) an ADN bottoms product that is substantially free of both BHMT and CPI.